Kowa 99 Update? (2 Viewers)

[paperweight]

I sold my 883 and bought a 99a in January 2022. I had compared my 883 to a 99a outside the shop in rather poor winter light and found the 99a better for my old eyes (then 71) because of the larger opening. I compared both scopes with the TE 11z I owned and the TE 80 which I later bought because I loved its performance in the test. I remember a distant tree with some leaves left, and the 99a really offered a better picture and more contrast.
I would never go back to the 883 although it was a fine scope. The reason is that I use the 99 for birding and nature exclusively and don't push it to its limits. I could since I own two Kowa 1.6 Extenders that could bring the 99a with the TE 11 to about 200x magnification which is about what is calculated for the limit of a scope (opening x 2)
For astronomy (planets and deep sky) I use my Takahashi TOA 130 with a Zeiss Mk V bino and Pentax plus Televise eyepieces now and then. All in all, my hobby is receding due to my age and I will sell all of my equipment sooner or later. My Swarovski EL 10x42 will do...

 

[Tringa45]

I think there are a few misconceptions here.
Resolution is an inherent quality of the objective lens(es) of a telescope or binocular.
The only relevance of magnification is the amount the individual user requires to be able to see its limits.
The limiting factor for the potential resolution is the diameter of the objective lens because diffraction occurs at its edges.
There are two empirical definitions of the resolution limit, from Rayleigh and Dawes.
The latter is defined as 116/D, where D is the objective diameter in millimetres. Accordingly, if a Kowa 99 were diffraction limited it should be able to resolve 1,17 arcseconds (as line pairs) and an 88 should be able to resolve 1,32".
In practice, with birding scopes and in particular binoculars, these limits are seldom met because of optical aberrations such as spherical and chromatic aberration, astigmatism or coma, and manufacturing defects.
The Kowa 883 or 88 has a rather "fast" focal ratio of f/5,7 making it very difficult to attain the Dawes' limit and even more difficult for the 99 with a similar focal ratio.
It is probably no coincidence that Juhani (BoldenEagle) and I both measured a resolution of 1,41" for our 883 scopes. That's probably as good as it gets for an 883. He used the extender for 96x and I used a 3,5 mm astro eyepiece for 140x.
A magnification of double the objective diameter in mm, as mentioned above, would result in a 0,5 mm exit pupil and would be empty magnification for those with good eyesight. Exit pupils this small would not only drastically reduce brightness but would also limit the resolution capabilities of the user's eye due to diffraction.

John

 

[paperweight]

@Tringa45: thanks for the helpful explanation. I only read about the formula (openingx2) somewhere and never really worked with high magnification, neither with the 883 nor with the 99a. Rarely do I employ one of the two Extenders I bought over the years. I once tried using both extenders on the 883 with the TE 10 eyepiece but did not find it worthwhile.
I usually use the 40x of the TE 80 or the max 70x of the TE 11. This will do with my "targets" (mainly birds).

 

[Binastro]

John,

You keep saying that 0.5mm exit pupils is empty magnification and the user's eyes can't use it.

This is just plain wrong.

It is an individual observation.

Most of the well known observers including Dawes habitually used 0.4mm exit pupils and some use higher powers with 0.3mm exit pupils.
These are also observers with good eyesight.
You just have to look at the recorded drawings that they made.

This is certainly not empty magnification, both astronomically and in favourable terrestrial limits.

Empty magnification is when increasing the magnification results in no more information being seen or indeed less information being seen.

One of the best planetary observers and excellent artist uses 200x and 300x with his 300mm Newtonian on Mars and Jupiter.
But when the Seeing is really excellent he goes up to 600x.

On a special night using the NASA 1 metre telescope on Pic du Midi two observers encountered very exceptional Seeing for about 5 hours.
They used 1,500x and 2,000x magnification and probably saw more detail than anyone has on Mars from the Earth.

Personally, and I don't have exceptional eyesight, I used 600x on my 317mm Dall Kirkham for Jupiter's moons.

Also the Pentax 100mm f/12 refractor easily took 300x, although Mars and Jupiter were getting dim.
At 400x this was not empty magnification for close double stars.

You also seem to forget or disregard the fact that William Herschel discovered Uranus using 250x, 450x and 900x with his 6.3 inch own made telescope. That is down to 0.16mm exit pupil.

I have also said repeatedly that at 3a.m. on many nights with 3 identical 120mm refractors at 250x I resolved detail on a clock tower at 4.7 miles that I had not seen before at lower magnification. One arcsecond resolution.
This is in a suburban location.

To see unknown detail one needs two or three times the minimum possible magnification.

The problem with test charts is that they are known detail.

Regards,
David

 

[Tringa45]

You keep saying that 0.5mm exit pupils is empty magnification and the user's eyes can't use it.
This is just plain wrong.

David,
The caveat was good eyesight, but even there I have my doubts.
Most of the examples you quoted involve magnifications involving exit pupils as low as 0,5 mm and I don't dispute their use. They may indeed have been in some way more comfortable for the observers but I don't believe that they provided any gain in information over somewhat lower magnificatios. You can't see more than the scope is capable of resolving.

As you know, I do not qualify as an amateur astronomer but I believe the biggest challenge involving resolution is the ability to resolve double stars.
This is the basis for Dawes' definition of resolution, where there is at least 5% reduction of brightness between two overlapping Airy disks.
These are high contrast targets and one can substitute a backlit 1951 USAF glass slide with increasing line pair frequency to measure the resolution.
The (theoretical) maximum resolution according to Dawes is even quoted by some astronomical scope manufacturers, e.g. Vixen.

If we were to take a diffraction limited scope with a 116 mm objective lens as an example it would be able to resolve 1 arcsecond expressed in line pairs. This corresponds to a line width of 0,5". An observer with 20/20 vision can resolve 1 arcminute, so would need 120x magnification to see its limitations. An observer with 20/10 eyesight would only need 60x, but an observer with 20/40 vision might profit from 240x. Here however the performance of the human eye is diminished by diffraction at 0,5 mm exit pupil.

In another thread, Hermann provided two quotes from Albert König and Horst Köhler:
"Wenn also ein Fernrohr eine Vergrößerung hat, daß sich eine Austrittspupille von 2mm ergibt, dann entspricht die Beugungsunschärfe gerade eben dem Auflösungsvermögen der Netzhaut. Man sagt, ein solches Fernrohr besitzt die "förderliche Vergrößerung". Es ist in vielen Fällen angebracht, die Vergrößerung weiter zu steigern, etwa so, daß die Winkeldistanz zweier getrennter Objekte etwa dem doppelten Auflösungsvermögen des Auges entspricht. Man wendet also die doppelte förderliche Vergrößerung an und erhält eine Austrittspupille mit einem Durchmesser von 1mm."

Effctively, 1 mm is the minimal useful exit pupil.

"Man kann das allenfalls noch bis zur 4fachen förderlichen Vergrößerung, d.h. bis zu einer Austrittspupille von 0,5mm Durchmesser treiben, man muss sich aber darüber im klaren sein, daß diese Steigerung bestenfalls der Bequemlichkeit des Beobachters, keineswegs aber einer weiteren Erkennbarkeit von Einzelheiten dient."

Effectively, one could go down to exit pupils of 0,5 mm for comfort but under no circumstances will it offer a gain in information.
There are similar statements here by Al Nagler: Choosing Your Telescope's Magnification where he mentions 20-30x magnification per inch of aperture for planetary observation, i.e. approximately 1 mm exit pupil.

Regards,
John

 

[Binastro]

You, maybe Hermann and maybe Al are just wrong.

I don't know Rev. Dawes actual eye prescription but he was called Eagle eyed.
If you look in November 2015, I think, Sky and Telescope you will see copies of his drawings with a 6.3 inch refractor.

There is a great difference between what observers actually use and what so called theory people may think they know.

Many of the best planetary observers use 65x to 70x per inch of aperture to finish their drawings and draw out the most information.
This is with very fine telescopes up to 6 or 7 inch aperture.

For 12 inch to 16 inch telescopes it is 50x per inch.

The maximum for any size telescope is usually about 1,000x or 1100x because of the limitations of the atmosphere.
Except for the rare Pic du Midi observation.

John, I will not let you get away with wrong information and so called theory.

There are several types of observation where high magnifications are needed to bring out unseen detail.

One is the observation of the faint moons of Saturn near the rings, another the moons of Uranus, Mars and maybe Neptune.
The high magnifications darken the sky and importantly increase the apparent distance from the planet or rings.

A recent observation is of Uranus moons with a 350mm Skywatcher Dobsonian at 650x. This is not the greatest telescope for this type of observation, but the moons were not seen at lower magnifications. This was from a dark site.

I saw Mars in Horace Dall's 8 inch Maksutov Cassegrain in his loft and through his worked plate glass window.
The magnification was 400x and the detail staggering. In this case 50x per inch.

If you look through the observations in ALPO magazine going back decades you will see similar high power drawings.
They are not made on a whim, but by experienced observers.

I repeat that there is no fact in saying 0.5mm exit pupils are the minimum the eye can use and even less that 1mm exit pupils can use.
These contentions are only made by non serious astro observers and their incorrect interpretation of theory.

There is also a skilled double star observer who accurately measures double star position angles and separations and routinely uses 800x with his fine 10 inch Newtonian.

Mars, in particular, takes high magnifications well when near the Earth and 18 arcseconds and larger diameter.

Facts are what are important not personal interpretations of so called theory.

Regards,
B.

 

[Hermann]

In another thread, Hermann provided two quotes from Albert König and Horst Köhler:
"Wenn also ein Fernrohr eine Vergrößerung hat, daß sich eine Austrittspupille von 2mm ergibt, dann entspricht die Beugungsunschärfe gerade eben dem Auflösungsvermögen der Netzhaut. Man sagt, ein solches Fernrohr besitzt die "förderliche Vergrößerung". Es ist in vielen Fällen angebracht, die Vergrößerung weiter zu steigern, etwa so, daß die Winkeldistanz zweier getrennter Objekte etwa dem doppelten Auflösungsvermögen des Auges entspricht. Man wendet also die doppelte förderliche Vergrößerung an und erhält eine Austrittspupille mit einem Durchmesser von 1mm."

Effctively, 1 mm is the minimal useful exit pupil.

"Man kann das allenfalls noch bis zur 4fachen förderlichen Vergrößerung, d.h. bis zu einer Austrittspupille von 0,5mm Durchmesser treiben, man muss sich aber darüber im klaren sein, daß diese Steigerung bestenfalls der Bequemlichkeit des Beobachters, keineswegs aber einer weiteren Erkennbarkeit von Einzelheiten dient."

Don't forget that these quotes refer to observations during the day ("Tagessehen"). König & Köhler stress that things are more complicated for astronomical obversations. However, they state again that an exit pupil of 0,5mm is the smallest that can be usefully employed. ("Die äußerste Grenze ist [...] der Durchmesser der Austrittspupille von 0,5mm, da bei kleineren Austrittspupillen die entoptischen Erscheinungen auftreten". (König/Köhler ³1959, p. 110) "Entoptische Erscheinungen" are e.g. floaters.

I don't doubt that some people with exceptional visual acuity may under some circumstances be able to use smaller exit pupils. However, they are anything but the norm.

My own experience with using well-corrected scopes for terrestrial observations is that an exit pupil of 1mm ("doppelte förderliche Vergrößerung") is a hard limit for me; once the exit pupil gets smaller things become too uncomfortable. I do have floaters in both eyes that are quite annoying even at exit pupils of ~1,5mm. I think it's no coincidence that the "old" Nikon Fieldscopes allowed 60x magnification with the 60mm scopes and 75x magnification with the 82mm scopes.

Hermann

 

[Tringa45]

You, maybe Hermann and maybe Al are just wrong.

Al Nagler, who designed optics for NASA and who has been a pioneer in the design of astronomical eyepieces is wrong?
Quote:- "Experienced planetary observers use 20x to 30x per inch of aperture to see the most planetary detail. Double star observers go higher, up to 50x per inch (which corresponds to a l/2-mm exit pupil). Beyond this, telescope power and eye limitations degrade the view."

I have just consulted "Telescope Optics" by Rutten & van Venrooij.
On magnification they state:-
"The greatest magnification possible is seldom the best for viewing a celestial object through a telescope. Experienced observers have established the guidelines for the diameter of the exit pupil fo different types of observing."
They then refer to a table:-
5-7 mm for wide fields under dark skies (large galaxies, Milky Way, diffuse nebulae, open clusters)

3-4 mm General viewing (nebulae, clusters)

2 mm Best match to eye's resolution (moon, globular clusters, planetary nebulae, double stars)

0,8 mm Maximum planetary detail (according to Texereau)

0,5 mm Close double stars under best skies

Regards,
John

 

[Hermann]

I repeat that there is no fact in saying 0.5mm exit pupils are the minimum the eye can use and even less that 1mm exit pupils can use.
These contentions are only made by non serious astro observers and their incorrect interpretation of theory.

Albert König and Horst Köhler were heads of the department for terrestrial telescopes and binoculars at Zeiss. König in the 1930s and 40s, Köhler after the war.

Hermann

 

[Tringa45]

I have just recalled the fim "Blow Up" in which the photorapher played by David Hemmings enlarges a 35 mm negative more and more in an attempt to see more detail.
The image would get larger and larger and correspondingly more and more blurred.
You can't see more than the resolution of the instrument allows.

 

[Binastro]

Any young astronomer who has used an old but good Japanese 80mm f/15 refractor knows that at 80x you are just beginning to see detail on Jupiter and Mars, but at 160x you see a lot of detail.

John, you are just using references to suit your fixed opinions..

As, I said earlier, look at the actual observations of experienced life long observers of the planets, and also double stars.
This goes back about two hundred years.

It is true that those with exceptional sight, say 20/8 or better need lower magnifications.

To say that 2mm is best for double stars is just daft.

And Al's comments about 20x to 30x per inch is also quite at odds with reality.
I know Al and had a long conversation with him.

As to planetary nebulae, these again take very well to very high magnifications.
Again, 2mm is daft unless one is talking about a few huge planetary nebulae.

I don't want to quote names of present top planetary observers, but as I said using a 300mm Newtonian he goes to 600x when Seeing permits. He has excellent eyesight.
And the two top Mars observers went to 2,000x on the Pic du Midi NASA 1 metre telescope.

There are other members on this forum who take to references that suit their arguments, without actually going to the real source.

Regards,
B.

 

[Binastro]

Terrestrial telescopes are another kettle of fish as one is looking through a horizontal layer of atmosphere.
Binoculars are another step down.

The film Blow up quote is again a red herring.

Go to the actual observers, including Dawes.

To start a planetary drawing one starts with a lowish power say 200x to 250x with a 12 inch scope depending on how good your eyesight is.
Then one goes higher to fill in detail.
If the Seeing is superb then one can go to a very high power to find anything previously missed.

Regards,
B.

 

[Tringa45]

I submit that those observers, just like the members on this forum who stack extenders, are fooling themselves and have not understood the optical restraints.
Or are the optical engineers all wrong?

Regards,
John

 

[Hermann]

I'd actually like to hear from people who use extenders with their scopes like Juhani and Kimmo, and from Henry, who can use high magnifications with his AP. These are interesting questions ...

Hermann

 

[Treerunner]

How much larger is the true field of view in %?
The ATX has larger AFOV?
According to producer specs are: 35-20m vs. 35-19m and 59-70º vs. 57-71º. However, I noticed that recent X eyepiece modules have larger FOV and zoom range - see improved X eyepiece modules i.e. you can gain TFOVs by updating the eyepiece module of the X - using the Swaro specs with my measures, it would result on 36.5m-19.4m!...
In terms of resolution, the X series benefit from higher mag extenders them Kowa - as I say at cr-telescopes, for similar image quality you will gain resolution increasing maximum magnification! I still have to update the site with the X115 and the mentioned info...

My ATX eyepiece is in for repair, so it will be a while before I can repeat any comparison tests. The Kowa 99 definitely has a larger FOV all the way from 30 to 70, maybe 10+% (over both the ATX 99 and 115) but as I say, I'll have to wait for the ATX repair to clarify. Not aware of any different versions of the ATX eyepiece? I thought the original was all there was?

 

[DRodrigues]

... Not aware of any different versions of the ATX eyepiece? I thought the original was all there was?

Producers usually don't announce small changes. Mine is a STX and the serial number is HA1122732A. Noticed the changes, comparing to an older STX and older ATX - never compared to a more recent X eyepiece module, but I think the differences are too big to result from sample variation.

 

[DRodrigues]

... I've had both my 115 and eyepiece back for free repairs. ...

What were the problems?
And let us know if were solved...
The AFOV / TFOV comparison with the 99 are also welcomed!...

 

[paperweight]

I've just done a series of comparisons. This is not a scientific approach and far from the theoretical discussions above. I'm just an amateur who likes birding and photography, that's all.

The object was a transmission tower about a mile away from my home. The sky was overcast, and there was no wind. For the Kowa 99, I used my Gitzo tripod and a Berlebach 160 ("Pegasus") head. For practical reasons, I shot from my porch which is not ideal because of the vibration. So I used a 10 sec automatic release on my iPhone 13 Pro Max and 1/500sec on my Sony camera.
The iPhone was put in a Smartoscope Adapter.

I started with my Sony RX 10 IV camera at about 35mm. The transmission tower is marked by an arrow. Autmatic ASA was 800, aperture 5.6 and shutter speed 1/500.






After that, I used the zoom lens of the Sony camera to its max of 600mm equivalent.




Again, ASA was 800, aperture 5.6 and shutter speed 1/500

Then came the iPhone itself with a 35mm equivalent shot. Again, I have marked the tower with an arrow. I did not really concentrate on focusing correctly, so the tower is out of focus. Focal length was 63mm equivalent, ASA 50, aperture 1.5 equivalent and shutter speed 1/738.



After that, I turned to the Kowa TSN 99a with the TE 11 eyepiece attached, installed the Smartoscope adapter with the iPhone 13 and used the highest zoom magnification (70x).




I find the data of iPhone images a bit erratic. The information about this one states an aperture of 5.7, a shutter speed of 1/147, ASA 32 (!) equivalent.
Then I installed the Kowa EX 16 extender (1.6x) on the TE 11 which results in a magnification of 112x. I guess I did not really focus well because the last picture below with 180x magnification is more in-focus than this one. It was cold outside, and I wanted to come in again as quickly as possible...






Finally I stacked both my EX 16 extenders which results in a magnification of 180x. I used the iPhone's step by step magnification to avoid vignetting which might be different in the two last pics (112 vs 180x).
Anyway this is the last picture with 180x magnification.




I know this is far from being scientific at all, I just wanted to give you an impression of how a Kowa TSN 99a works with different steps of magnification compared to a good camera (which I think the Sony RX 10 IV is).

 

[Tringa45]

@paperweight:
With that last picture I think you can dismiss any doubts about the quality of your 99A.
Btw, your camera and probably no other commercially available camera has a chance against the Kowa at this distance.

John

 

[paperweight]

Thanks @Tringa45 for the nice comment. At almost 74 my interest in all my hobbies (music, photography, astronomy) is slowly but firmly fading.
The iPhone can take 180x with daylight situations, but my eyes can't. In other words, I don't enjoy watching at this rate of magnification. Having done such a test, I have doubts about selling the whole system (including the TE 80 and the neoprene bag). I offered it over here a couple of times but will now ask my ophthalmologist for advice. He told me two years ago that my cataract may ask for a surgery in a couple of years. If my visus can be improved, I might consent to a surgery sooner or later.